This invention relates to tags for article surveillance systems and, in particular, to tags for magnetic article surveillance systems.
There have been developed over recent years many types of tags for use in magnetic article surveillance systems. In particular, there have been developed a number of so-called "passive" tags which do not require internal power, but which can generate a sensible signal when subjected to a magnetic field. "Passive" tags are considered advantageous because they are less expensive, have longer life and are lighter than tags which utilize internal power sources.
U.S. Pat. No. 4,622,543 discloses one type of magnetic tag in which the tag is formed from a magneto-strictive ferromagnetic strip encased within a hard magnetic container. The magnetic container is adapted, upon being magnetized, to arm the ferromagnetic strip so that it mechanically resonates at a preselected frequency.
When the tag of the '543 patent passes through an interrogation zone in which magnetic energy is transmitted at or near the resonant frequency, the magneto-strictive strip mechanically resonates or vibrates at the resonant frequency. This vibration produces an acoustic signal which is received by an acoustic receiver at a location remote from tag. The receiver processes the acoustic signal and generates an alarm, indicating presence of the tag and its corresponding article in the zone.
While the tag of the '543 patent provides the desired indication that a tag is passing through an interrogation zone, it is sometimes desirable to be able to more closely determine the exact location of the tag in the zone. U.S. Pat. No. 4,573,042 discloses a tag which allows such closer determination by itself emitting an audible sound when in an interrogation zone. The tag of the '042 patent, however, is an active tag adapted for use with radio frequency interrogation signals and is not readily adaptable to magnetic systems.
It is, therefore, a primary object of the present invention to provide an improved tag for a magnetic article surveillance system.
It is a further object of the present invention to provide a passive tag for a magnetic article surveillance system which is able to provide an indication of the location of the tag in the interrogation zone of the system.
It is a further object of the present invention to provide a passive tag for a magnetic article surveillance system in which the tag itself emits an audible signal for location identification.